The rendering of verse 17 is doubtful, but the general meaning is clear.
The brave speakers have hope that God will rebuke the king's taunt, and
will prove Himself to be able to deliver out of his hand. So they repeat
his very words with singular boldness, and contradict him to his face.
They have no absolute certainty of deliverance, but whether it comes or
not will make no manner of difference to them. They have absolute
certainty as to duty; and so they look the furious tyrant right in the
eyes, and quietly say, 'We will not serve thy gods.' Nothing like that
had ever been heard in those halls.

Duty is sovereign. The obligation to resist all temptations to go
against conscience is unaffected by consequences. There may be hope that
God will not suffer us to be harmed, but whether He does or not should
make no difference to our fixed resolve. That temper of lowly faith and
inflexible faithfulness which these Hebrews showed in the supreme
moment, when they took their lives in their hands, may be as nobly
illustrated in the small difficulties of our peaceful lives. The same
laws shape the curves of the tiny ripples in a basin and of the Atlantic
rollers. No man who cannot say 'I will not' in the face of frowns and
dangers, be they what they may, and stick to it, will do his part, He
who has conquered regard for personal consequences, and does not let
them deflect his course a hairsbreadth, is lord of the world.

How small Nebuchadnezzar was by the side of his three victims! How empty
his threats to men who cared nothing whether they burned or not, so long
as they did not apostatise! What can the world do against a man who
says, 'It is all one to me whether I live or die; I will not worship at
your shrines?' The fire of the furnace is but painted flames to such an
one.